This paper describes a new determination of the primordial helium abundance ( Y _ { P } ) , based on the abundance analysis of five metal-poor extragalactic H ii regions .
For three regions of the sample ( ( catalog SBS~0335–052 ) , ( catalog I~Zw~18 ) , and ( catalog H~29 ) ) we present tailored photoionization models based on improved calculations with respect to previous models .
In particular , we use the photoionization models to study quantitatively the effect of collisional excitation of Balmer lines on the determination of the helium abundance ( Y ) in the individual regions .
This effect is twofold : first , the intensities of the Balmer lines are enhanced with respect to the pure recombination value , mimicking a higher hydrogen abundance ; second , the observed reddening is larger than the true extinction , due to the differential effect of collisions on different Balmer lines .
In addition to these effects , our analysis takes into account the following features of H ii regions : ( i ) the temperature structure , ( ii ) the density structure , ( iii ) the presence of neutral helium , ( iv ) the collisional excitation of the He i lines , ( v ) the underlying absorption of the He i lines , and ( vi ) the optical thickness of the He i lines .
The object that shows the highest increase in Y after the inclusion of collisional effects in the analysis is ( catalog SBS~0335–052 ) , whose helium abundance has been revised by \Delta Y = +0.0107 .
The revised Y values for the five objects in our sample yield an increase of +0.0035 in Y _ { P } , giving Y _ { P } = 0.2391 \pm 0.0020 .